This is very interesting to reconsider this question over a year following this post I made in this thread way-back-when. In thinking about this again, I still believe most of what I wrote back then.
I still stand by the idea that Ennis was making progress (at a snail's pace) in certain ways in terms of understanding and accepting his relationship with Jack. We see that, clearly, Jack's death was a shock strong enough to rattle Ennis into doing all sorts of things that he would have been too afraid to do prior to Jack's death (primarily driving all the way to Lightning Flat to meet Jack's parents... and even gathering his courage to call Lureen, etc.). I wonder if the lake-side fight alone was enough to rattle Ennis to the degree that he really would have tried harder to make things better with Jack.
I do think it's extremely significant that Ennis is the one who sent the final postcard. Clearly the thought of losing Jack is tremendously profound to Ennis. We see this in his reactions to the conversation by the lake when he begins to worry about a break-up and obviously we see what happens in the circumstance where he does lose Jack. So, maybe if Jack had lived he really would have tried harder. I think Jack's hints and not-so-subtle threats about breaking up during the argument really could have had a major impact on Ennis's behavior and determination to fix things.
One of the last things we hear Ennis saying to Jack is "I can't stand this anymore"... So, according to Ennis's own motto "if you can't fix it, you've got to stand it"... this seems to suggest that once you can't stand something anymore you do the opposite and try to fix it. There's no exit clause in his motto. If you can't stand something you still don't quit it.
As far as Jack goes, I think he's self-aware enough to know that he's not capable of breaking up with Ennis. I think that's why he adds the conditional "wish" to "I wish I knew how to quit you." I don't think he has the will power to resist Ennis and I think he knows that. If he had lived, I think he would have worked with Ennis to try to make things better. Like I said in my post a year ago, I think progress would have been in baby-steps. Mostly I think things would have stayed the same... at least in the short term.
But, again, I think the argument alone... and airing all the frustrations and some of the secrets... was enough to be a catalyst for some measure of change.
The point you make about Ennis when he says, I can't hardly stand it, is interesting, because as you say ,in his own words if you can't stand something you have to fix it.
Also in the S.S Jack is not sure whether Ennis collapsing, is a heart attack or rage.It is clear that the scene is a culmination of 20 years of things glossed over and never spoken about.
Right at the end of that scene in the SS Jack is thinking ,let be, let be.Directly after he remembers the dozy embrace which so captivates him.So I would guess he is now thinking anything is better than no Ennis.Perhaps the thought that briefly crosses his mind that Ennis has had a heart attack,shocks him back, to once again realising how deep and desperate is his love for him, hence the let be let be.Don't rock the boat and risk loosing him completely.
I certainly think they would have continued together.I don't think Jack, even if he had tried with Randall would have continued.
There is no one in his heart who can replace Ennis.
In real life when we try to do that,the person we use as a substitute generally just reminds us even more of the person we have lost, or given up on.
I wish I could believe that they would eventually moved in together,I am just not sure. In my mind this is where fact and fiction become joined. Even now parts of America are still so homophobic,I struggle to see them together under one roof.
If they had gone down that route, they sure as hell would have to have moved states, to a more liberal one.The problem there is the more liberal states would not have provided Ennis with any work that he enjoyed,and I can't see him living off Jack's money. That would just be a step too far.